Planning a basement 22/6/2018. Elaine Toogood Senior architect The Concrete Centre. June

Transcription

1 Planning a basement Elaine Toogood Senior architect The Concrete Centre June Aims to encourage and enable the design and construction of quality basements on new developments and the improvement of existing and retro-fit basements. A forum to Develop guidance documents to simplify design and enable high performance basement construction. Provide information on products and materials for use in basement design and construction. To encourage good practice in design and construction through promotion of guidance. 1

2 Contents Image courtesy of LTS Architects Benefits Types of basements Process of design and approvals Research Planning Site investigations Building Control approval Other considerations Concrete construction Party wall awards Further guidance Definition of basement A usable part of a building that is situated partly or entirely below ground level British Standards Institute A storey at least 1.2m below adjoining ground level Building regulations in England and Wales 1.2m 2

3 Basements in the UK Basements common in the UK before changes to house construction in 20th Century: Cool food storage Domestic help Fuel storage Common in America/Canada and mainland Europe Recent resurgence in basement construction Benefits of basements for home owners Include: Additional storage More rooms Added value to property Improve don t move culture Expanded families (adult children/older parents living at home) Self contained accommodation that could generate rental income Home working Space for heating/cooling equipment e.g. ground source heat pump cylinder Space for leisure activities (home gym/home cinema) Future adaptability 3

4 Performance benefits of basements Include: Good acoustic isolation Energy efficient construction amount of insulation needed below ground is less than above Inherently good airtightness High thermal mass potential Benefits of basements for development Effective use of sloping sites Efficient use of poor ground conditions Added value (potential to unlock a sites development viability) More accommodation in areas with development height or footprint restrictions Efficient land utilisation/ greater density Protection of amenity space 4

5 Benefits of basements for development A basement garage requires less street frontage Reducing size of footprint can enable greater density of development Types of basements for housing Refurbishment Extending an existing cellar Retrofit New basement under existing property New build Basement built during construction of the house Garden basement Attached or detached 5

6 Refurbishment E.g extending an existing cellar Works likely to include: Lowering floor level and underpinning walls Structural and waterproofing repairs New/additional waterproofing system Improving ventilation Consider new daylighting Improving access/ means of escape Planning permission may not be required Listed building consent may be required Building Control approval is required Retrofit New basement under existing property Specialist work Heavily engineered Support/protection of existing and neighbouring properties and removal of soil are key considerations Maybe key to unlocking potential of site development Planning permission likely to be required Listed building consent may be required Building Control approval is required 6

7 New build Basement built as part of new house construction Opportunity for optimum design solutions as considered at the outset Cost effective construction (simpler access/ economies of scale) Option to build a utility spec i.e. unfurnished space for future conversion Planning permission and Building Control approval carried out as part of whole development Garden Basement New basement in existing garden New below ground structure with planted green or landscaped roof to retain accessible outdoor space Could be connected to property or separate Need for structural support of existing house reduces as distance form it increases Planning permission likely to be required Listed building consent may be required Building Control approval is required 7

8 Garden Basement Garden level Basement space below Design and approvals process Essentially the same design process for basements as any housing development in terms of statutory approvals and design process but there are specific issues impacting design development These vary according to type of basement being planned and location i.e. Planning permission (likely) Building control approval (required) Other potential approvals include: Landlords consent, Listed Building Consent, Highways, Building Warranty (e.g. NHBC) 8

9 RIBA plan of works Source: Architects Journal May 2013 Industry standard plan Guidance for clients and architects provided by RIBA Simplified design process for basements pre-planning Desk-top research/ investigations Concept Design Physical site investigations Detailed design for planning application Establish design parameters Statutory and Site context Brief (use of basement) Produce design concept based on site investigations & brief Obtain preplanning advice Consider Building Control issues effecting layout Physical investigations to inform detailed design E.g soil & ground water conditions, contaminants Develop proposals in more detail Waterproofing expert and Structural engineer input 9

10 Desk top site analysis/research Site survey- including topology/ drainage/services Flood risk assessment Ground Geology Archaeology Local Authority planning guidelines Establish basement use/ potential use Establish if any easements/or restrictive covenants Location and type of trees/ TPO s Desk top site analysis/research 10

11 Planning permission Formal permission from the local authority for the erection or alteration of development The planning regime covering the creation of living spaces in basements is evolving and under review Planning permission for basements are likely to be required for Basements that are part of a new development above ground New excavations which involve major works A new separate unit of accommodation Development that alters the external appearance of the property, such as adding a light well Planning guidelines vary between Local Authorities and pre-application advice recommended Permitted Development In some cases a basement can be built as permitted development if: Single storey Under the footprint of the original dwelling No greater than 3m in depth Converting an existing cellar or basement into a living space unlikely to require planning approval, unless you are altering the visual performance of the property e.g. light well General Permitted Development Order is updated regularly, it is advised that specialist advice is taken 11

12 Location specific planning requirements Planning guidelines vary between Local Authorities and preapplication advice recommended Some supplementary planning documents (particularly in central London) require additional information as part of planning application e.g. structural engineers report B Extract from Figure 1, Basement Development in Westminster

13 Building Regulations Minimum standards for design, construction and alterations. Application for approval made to Local Authority building control department or an Approved Inspector. Building Regulations apply to all basement construction Advisory to submit for full plans approval, i.e. deposit plans in advance. Building regulations guidance Practical guidance to help meet building regulation requirements for basements for dwellings and other good practice guidance. Site preparation Structure Fire safety Conservation of fuel and power Ventilation Resistance to the passage of sound Drainage and waste disposal Heat producing appliances Access and facilities for disabled people Vehicle access 13

14 Building regulations guidance Based on former Approved Document Basements for dwellings (2010 regs) and 2013 addendum. Design guidance fire safety Habitable rooms in a basement require: An external door or window suitable for escape Or A protected escape stair Figure 4.2 Inner room arrangements are possible for: kitchens & utility rooms dressing rooms & bathrooms 14

15 Design guidance fire safety Figure 4.5 Escape into an enclosed garden is acceptable if: The depth of the garden exceeds the height of the house above ground level Or The height of the rear extension (whichever is the greater) Design guidance ventilation Figure 6.3 Figure 6.4 Separate basement dwellings or those without permanent opening to above ground dwelling is treated as a single story dwelling above ground = Increase background ventilation by 10,000m 2 15

16 Design guidance ventilation Figure 6.6 Non-habitable basements (cellars) can be constructed without ventilation or windows but consideration of condensation and vapour control may be needed. Consider future possible changes of use as they may need full ventilation Design guidance - structure A retaining wall should not be less than 1.5 x the depth of a basement away from a road or other trafficked area (except for a drive or parking area solely serving an individual dwelling) Figure

17 New building warranty requirements e.g. NHBC Technical requirements and recommendations for the waterproofing of basements and other structures below (or near to) ground level necessary to obtain NHBC Building Warranty. New NHBC Technical Standards issued Physical site investigation E.g. core samples or trial pits & CAT scans Soil type/conditions e.g. permeability, presence of natural gases or aggressive materials Ground water & direction of flow Water table plus seasonal variations Location of subterranean services etc Location of ground drainage Movement risks potential subsidence Boundary conditions 17

18 Detailed design waterproofing and construction strategy Type A: Barrier system Type B: Structural integral protection Type C: Drained protection 3 main types of protection against ground water Influenced by ground conditions and building use Concrete wall systems Masonry Cast in situ Precast units Contemporary basements are constructed using concrete Selection of system is influenced by type of waterproofing strategy adopted Twin wall 18

19 Concrete wall systems Hollow masonry Insulated concrete formwork Twin wall Permanent formwork systems for cast insitu concrete Concrete interim floor Beam and block floors Hollow core or solid precast Cast insitu or lattice girder (hybrid) Performance benefits of concrete include: Sound insulation Fire resistance and isolation Single span (flexible space planning) Simple detailing with retaining walls Flood resistance 19

20 Concrete for deep basement construction Concrete lining wall to sheet piles King Post Wall Contiguous Piled Wall Simplified design process for basements post planning Detailed design for tender Building Control submission Party Wall notices Construction Develop detailed drawings & specification for tender E.g. insulation/ reinforcement calculations/ waterproofing details Full plans submission is advised to obtain approval of plans before construction starts Serve notice before construction begins Ideally using experienced contractor Building Control inspects during works 20

21 Detailed design - conservation of fuel and power Figure 5.1 Calculating the average basement depth U values for individual wall elements can be calculated individually or using an average depth below ground Detailed design - conservation of fuel and power Tables for determining thickness of insulation to achieve given U-values in different ground conditions 21

22 Party Wall agreement Governed by Party Wall Act 1996 The Party Wall Act exists to protect the concerns of neighbouring landowners and to facilitate an agreement between them with regards to construction works A party wall notice is issued to adjoining owners at least one month before works start. Usually conducted by a chartered surveyor. The Party Wall agreement sets out brief details of the proposed works and related matters such as The current condition (and photographs) of the adjoining owners property) Working hours for construction to take place A date for starting and finishing the works The details of the contractor s public liability insurance Access arrangements for surveyors Indemnities by the building owner in favour of the adjoining owner Party Wall agreement Required when excavation and construction of foundations and basement walls are within 3m of a neighbouring structure Less tan 3m Adjoining Owner Building owners excavation 22

23 Party Wall agreement Less than 6m Required when excavation is : Extended deeper than the neighbouring structure s foundations Within 6m of the neighbouring structure and lower than a line drawn 45 o from the bottom of the neighbours foundation Adjoining Owner 45 Building owners excavation Summary Lots of reasons and benefits to build basements Design process is same as any building works, with a few specific considerations Appropriate site investigation and research is essential Early collaboration with a Structural Engineer and Waterproofing Expert is essential 23

24 Further Guidance Further Guidance 24

25 THANK Image courtesy of LTS Architects THANK 25